This invention relates to a photoconductive image pick-up tube of the type wherein photoelectric conversion signals are read by an electron beam scanned at a high velocity.
In the past, most image pick-up tubes are based on a low velocity electron beam scanning positive charging system (hereinafter referred to as LP system) in which an electron beam is scanned at a low velocity, and therefore have a long lag especially at low illumination and suffer from beam being responsible for tendency to the generation of image distortion and shading.
Incidentally, an expedient to solve the above disadvantages has been known as a high velocity electron beam scanning negative charging system (hereinafter referred to as HN system) as disclosed in Japanese patent application Laid-Open No. 54-44487 laid open on 1979, for example. FIG. 1 is useful in explaining the operational principle of the HN system. According to the HN system, a transparent electrode 2 is applied with a positive voltage relative to a cathode electrode 9 so that a target of the image pick-up tube possesses a secondary electron emission yield .delta. which is greater than 1 (one). When an electron beam 6 is scanned at a high velocity under this condition, a secondary electron beam 7 is emitted from the surface of the target (especially a photoconductive layer 3) and as a result, a potential at the target surface is brought into equilibrium with a potential at a focusing collector electrode 8 to become positive relative to the transparent electrode 2. Light passing through a transparent substrate 1 and the transparent electrode 2 is absorbed by the photoconductive layer 3 to generate electron-hole pairs of which electrons are drawn in the direction of scanning surface to cause the surface potential to drop toward negative. This potential drop is derived as a signal. In FIG. 1, reference numerals 10 and 11 denote a load resistor and a target voltage source, respectively.
This HN system image pick-up tube is superior to the conventional LP system in that the capacitive lag is reduced, that the resolution at the corner is high, and that the image distortion is reduced. In the proposed HN system, however, the potential difference between the focusing collector electrode 8 and the scanning surface of the target is almost zeroed with the result that the secondary electron emitted from the target is partly scattered toward the target surface to redistribute thereon, giving rise to the generation of spurious signals and consequent impairment of high quality image.
A countermeasure for elimination of the spurious signals has been proposed (U.S. patent application Ser. No. 547,962 filed on Nov. 2, 1983, now U.S. Pat. No. 4,556,817) wherein an electrode for defining an additional potential is disposed near the target to cause the secondary electron to be collected forcibly by the collector electrode 8, and the electrode for defining the additional potential is materialized by a metallic mesh-like electrode directly applied to the beam scanning surface of the target or by a stripe-like balance electrode formed on the scanning surface. This proposal on the one hand succeeds in suppressing the redistribution but on the other hand suffers from degradation of characteristics including degradation of resolution and increase in lag. Accordingly, the advent of an image pick-up tube has been desired which can be free from adverse secondary effect so as to eliminate the spurious signal.